An ongoing challenge in the optical networks industry is to maximize the data transmission capacity of a single fibre. The introduction of multiple channels through Wavelength Division Multiplexing (DWDM) systems has greatly improved this capacity, as has the steady improvement of the data bit rate on a single channel As a result, there are now three alternative paths to follow for increasing overall data transmission, namely increasing the single channel bit rate, reducing the channel spacing or increasing the optical bandwidth.
In an optical looped network the optical signals are added and dropped at the network nodes and then the dropped signals are further transmitted in different part of the network e.g. towards subscriber. The added signals, in turn, are multiplexed with the signals in the looped part of the optical network. The distances between the nodes of the network are long enough to attenuate the optical signals in addition to the attenuation experienced at the add/drop multiplexers and other passive parts of the network. To compensate this attenuation the optical signals are amplified.
One of the main problems to be faced when designing amplified optical rings in closed form for use with WDM applications is recirculation of Amplified Spontaneous Emissions (ASEs) produced by each amplifier, usually of the rare earth doped fibre type, for example, especially an Erbium-Doped Fibre Amplifier (EDEA).
WDM ring structures usually employ filters for adding or extracting specific channels from the optical line. In many cases to compensate for losses in the fibres or filters one or more optical amplifiers are necessary along the ring.
The noise produced by these amplifiers outside the band allocated for the channels can recirculate in the ring if not controlled. If overall gain on the network ring is more than one, i.e. total gain is greater than total losses as may happen if some amplifier amplifies more than the attenuation of the preceding section, the ASE noise could be amplified as a signal and grow indiscriminately in the ring because of recirculation, making it difficult to control the ring status and ensure survival of the traffic channels.
The known ways of solving the problem include introducing an interruption along the ASE noise path at some point on the ring. Wavelength blocker enables to remove the residual noise and noise generated out of the bandwidth. In this manner the problem is solved with the disadvantage of having to introduce additional passive components and/or with the loss of flexibility in the system. Centralized traffic is necessary or any traffic reconfiguration requires the visit of the node which realizes the ASE interruption.
In the second approach each the amplifiers installed in the optical ring (loop) requires rather expensive Power Monitoring Unit. This approach tends to force the network ring gain to keep it below the lasing threshold so that the ASE recirculation cannot increase in power while propagating along the ring. A problem with this approach is that EDFA or similar amplifiers have gain which depends on the power applied at input and in the power grid the power input to the amplifiers depends in turn on the number of channels active at that time. For this reason, in order to keep total gain beneath the lasing threshold under all possible conditions, including the addition or removal of channels and nodes, a complex global control algorithm of the ring with many monitoring points is needed or else it is necessary to hold the gain of the individual amplifiers low enough to ensure that even under conditions leading to the highest gain of the amplifiers the total gain in the network is less than 1. But this solution involves a considerable reduction in the overall performance achievable since when far from the highest gain conditions amplification of the individual amplifiers is much lower that that which could be realized.
In another known solution, disclosed in WO2004/064280, for a looped optical transmission system with rare earth doped fibre optical amplifiers (e.g. EDFA) a gain control is achieved by positioning a gain peak at a wavelength (λASE) outside the band (λ1-λN) of the channels transmitted along the ring. This gain peak at λASE corresponds to an ASE emission peak of the amplifiers in the ring and the method employs the lasing peak produced in this way as a gain stabilization signal. Recirculation of the ASE noise is subject to peak gain effects which produce a peak lasing at λASE (as stated, outside the band λ1-λn reserved for the channels). This peak lasing, usually considered harmful in other systems, can be used as a signal for stabilizing the gain of all the amplifiers in the ring, hence achieving a ring with blocked gain with the stabilization signal, which is not inside a particular individual amplifier but is common to all the amplifiers in the ring. In this solution all the amplifiers of the ring are controlled and stabilized by increase and control of the ASE noise lasing peak.
A disadvantage of the solutions known in the art is that the ASE peak covers part of the spectrum that could be successfully used for transmission channels (i.e. part of the spectrum between 1530 nm and 1560 nm). At present the λ1-λn range is enough to accommodate less than 32 channels that are spaced by 100 GHz as provided by the ITU recommendation.
Hence, an improved optical amplifier would be advantageous and in particular one that can transmit more communication channels in wider spectrum without use of expensive Power Monitoring Units.